1. Field of the Invention
The present invention relates to optical communications apparatuses and, more specifically, to an optical communications apparatus for transmitting an optical signal by switching optical communications paths based on the wavelength and modulating frequency of the optical signal as address information.
2. Description of the Background Art
FIG. 10 is a block diagram showing the structure of a conventional optical communications apparatus. One example of such apparatus is disclosed in detail in xe2x80x9cHyperspace Addressed Optical Access Architecture using Active Arrayed Waveguide Gratingsxe2x80x9d, F. Farjaday, M. C. Parker, and S. D. Walker, OECC98, 15A2-2, 1998.
In FIG. 10, the optical communications apparatus includes an optical transmitting circuit 10001, a main optical transmission path 1004, an optical router 1005, first and second distribution optical transmission paths 10061 and 10062, and first and second optical receiving circuits 10091 and 10092. The optical transmitting circuit 10001 includes an address extractor 1010 and a variable wavelength optical modulator 1003.
In the above structured optical communications apparatus, the address extractor 1010 extracts, from a signal received by the optical transmitting circuit 10001, address information indicating the destination to which the signal should go. Alternatively, the address extractor 1010 may be provided with the address information itself separately.
The variable wavelength optical modulator 1003 is composed of a variable wavelength light source capable of changing the wavelength of output light. This wavelength is uniquely determined based on the address information extracted by the address information extractor 1010 or separately provided. The variable wavelength optical modulator 1003 optically modulates the signal including the above described data information, and then sends out light having the determined wavelength to the main optical transmission path 1004.
The optical router 1005, exemplarily composed of an AWG (Arrayed WaveGuide), has a plurality of output terminals (in this example, first and second output terminals 10051 and 10052) for selectively outputting the optical signal based on the wavelength of the input light. When supplied with the optical signal through the main optical transmission path 1004, the optical router 1005 outputs it from the first terminal 10051 when the optical wavelength thereof is xcex1, while outputting from the second terminal 10052 when xcex2.
The first and second optical receiving circuits 10091 and 10092 are each connected to the optical router 1005 at the first output terminal 10051 and at the second output terminal 10052, respectively. The first and second optical receiving circuits 10091 and 10092 each convert the optical signal from each corresponding output terminal into an electrical signal for output.
As described above, in the conventional optical communications apparatus, a variable wavelength light source is used as the light source in the optical transmitting circuit to control the wavelength of the output light based on the address information indicating the destination to which the data information should go. Also, the optical router is provided on the optical transmission path, enabling routing of the input light for output from each different terminal based on the wavelength thereof. Thus, the conventional optical communications apparatus can carry out autonomous switching among the transmission paths in optical domain, and therefore a high-speed optical communications network can be achieved.
One disadvantage here is, when the wavelength of the optical signal is used as an address, the number of wavelengths or wavelength bands available on the optical transmission path is limited. This disadvantage is described below with reference to FIG. 11.
FIG. 11 is a schematic diagram demonstrating the limitation of the number of wavelengths in the conventional optical communications apparatus. Specifically, as shown in FIG. 11, Erbium-doped fiber optical amplifiers (EDFA) widely used in optical transmission systems can generally carry out amplification only within approximately 30 to 40 nm in a wavelength band of 1.55 xcexcm. On the other hand, AWGs and optical filters generally have a wavelength resolving power (dividable optical wavelength period) of approximately 0.8 nm. In FIG. 11, the band pass characteristics of the optical filter is represented as a dotted line. Consequently, the number of wavelengths available in address space is very limited, approximately 40 to 50. Thus, in the conventional optical communications apparatus, the number of optical receiving terminals that can be connected thereto or covered thereby (the number of subscribers) is disadvantageously limited, and a large optical communications network cannot be constructed.
In order to construct a large optical communications network using the conventional optical communications apparatus, one structure can be suggested, where further routing is made using electrical signals outputted from the first and second optical receiving circuits 10091 and 10092 for transmitting information to end receiving terminals (subscribers). In such structure, however, unauthorized information extraction and tampering are highly possible due to the use of the electrical signals for information transmission to the end receiving terminals (subscribers), compared to the case where optical signals are used. Also, conventional communications networks using electrical signals are inferior, in transmission speed and amount of transmittable information, to optical communications networks using optical signals for transmitting information up to end users.
Therefore, an object of the present invention is to provide an optical communications apparatus achieving a large optical communications network with high speed and security by using the wavelength of an optical signal as an address for switching among transmission paths in optical domain.
The present invention has the following features to achieve the object above.
A first aspect of the present invention is directed to an optical communications apparatus for optically transmitting a transmission signal including data information a destination, and the apparatus includes:
a variable frequency RF modulator for modulating the transmission signal into an RF modulated signal, with a predetermined carrier frequency that corresponds to a lower address of address information uniquely set to the destination, the lower address representing the destination in a predetermined group to which the destination belongs;
a variable wavelength optical modulator for modulating the RF modulated signal outputted from the variable frequency RF modulator into an optical signal, with a predetermined optical wavelength that corresponds to an upper address of the address information, the upper address representing the predetermined group to which the destination belongs;
an optical router provided with a plurality of output terminals, for selectively outputting the optical signal outputted from the variable wavelength optical modulator from one of the output terminals that corresponds to the wavelength of the optical signal;
a plurality of RF optical routers each provided with a plurality of output terminals, for selectively outputting the optical signal coming from the output terminal of the optical router from one of the output terminals that corresponds to the carrier frequency of the RF modulated signal on the optical signal; and
a plurality of optical receiving circuits each for converting the optical signal outputted from the corresponding output terminal of the RF optical router into an electrical signal that corresponds to the transmission signal.
In the first aspect, by using the structure capable of selecting a signal transmission route in optical domain for switching (routing), the optical wavelength is related to the upper address of the address information indicative of the signal destination, and the (carrier) frequency of the RF modulated signal is related to the lower address. Based on the optical wavelength, a first optical routing is carried out, and then, based on the RF modulating frequency, a second optical routing is carried out. Thus, a large-capacity, high-speed optical communications apparatus capable of covering more optical receiving terminals can be achieved.
According to a second aspect, in the first aspect, the apparatus further includes an address extractor for extracting the address information from the transmission signal including the address information, and outputting the lower address to the variable frequency RF modulator and the upper address to the variable wavelength optical modulator
In the second aspect, the transmission signal further includes address information in addition to data information. Therefore, by extracting the address information from the transmission signal for optical routing, the optical communications apparatus does not have to be separately supplied with the address information.
According to a third aspect, in the first aspect,
the variable frequency RF modulator is plurally provided, each converting the transmission signal to a different destination into the RF modulated signal with different carrier frequency,
the variable wavelength optical modulator is plurally provided, each converting the RF modulated signal outputted from the corresponding variable frequency RF modulator into the optical signal, and
the optical router is supplied with the optical signals from all variable wavelength optical modulators as being multiplexed.
In the third aspect, optical signals coming from a plurality of optical transmitting circuits are multiplexed, and in the optical spectrum of the multiplexed optical signal, a transmission route is selected based first on the optical wavelength, and then on the RF modulating frequency. Thus, the optical transmission path is more efficiently used, and a high-speed, large-capacity optical multiplex communications apparatus can be achieved.
According to a fourth aspect, in the first aspect,
the variable wavelength optical modulator carries out optical intensity modulation,
the variable frequency RF modulator carries out ASK (Amplitude Shift Keying) modulation,
each of the RF optical routers includes:
an optical brancher for outputting the optical signal from a plurality of output terminals; and
a plurality of optical modulators each for subjecting the optical signal outputted from the corresponding output terminal of the optical brancher to optical intensity modulation with a signal having a frequency equal to the carrier the predetermined frequency of the RF modulated signal, and
each of the optical receivers includes:
a square-law-detector for carrying out square-law-detection on the optical signal outputted from the corresponding RF optical router, and outputting an electrical signal; and
a filter for passing a predetermined low frequency component of the electrical signal outputted from the square-law-detector, and outputting baseband information of the RF modulated signal.
In the fourth aspect, optical intensity modulation is used as the optical modulation scheme, and ASK modulation is used as the RF modulation scheme. The optical signal is modulated with the frequency corresponding to the RF modulated signal to be extracted, square-detected by the optical receiving terminal, and then baseband information of the RF modulated signal is reproduced for routing in the optical domain based on the RF modulated frequency. Thus, a larger, higher-speed optical communications apparatus can be achieved.
According to a fifth aspect, in the first aspect,
the variable wavelength optical modulator carries out optical intensity modulation,
each of the RF optical routers includes:
an optical brancher for outputting the optical signal from a plurality of output terminals; and
a plurality of optical filters each for extracting, from the optical signal outputted from the corresponding output terminal of the optical brancher, an optical carrier component and a double sideband component corresponding to the predetermined frequency of the RF modulated signal, and
each of the optical receivers includes:
a square-law-detector for carrying out square-law-detection on the optical signal outputted from the corresponding RF optical router, and outputting the RF modulated signal.
In the fifth aspect, optical intensity modulation is used as the optical modulation scheme. From the optical signal, the optical carrier component and the double sideband component corresponding to the RF modulated signal to be extracted is passed and extracted, square-detected by the optical receiving terminal, and then the RF modulated signal is reproduced for routing in the optical domain based on the RF modulated frequency. Thus, a larger, higher-speed optical communications apparatus can be achieved.
According to a sixth aspect, in the first aspect,
the variable wavelength optical modulator carries out optical intensity modulation,
each of the RF optical routers includes:
an optical brancher for outputting the optical signal from a plurality of output terminals; and
a plurality of optical filters each for extracting, from the optical signal outputted from the corresponding output terminal of the optical brancher, double sideband components corresponding to the predetermined frequency of the RF modulated signal, and
each of the optical receivers includes:
a square-law-detector for carrying out square-law-detection on the optical signal outputted from the corresponding RF optical router, and outputting a signal component that is a multiplied component of the RF modulated signal.
In the sixth aspect, optical intensity modulation is used as the optical modulation scheme. From the optical signal, the double sideband components corresponding to the RF modulated signal to be extracted are passed and extracted, square-detected by the optical receiving terminal, and then the RF modulated signal is reproduced for routing in the optical domain based on the RF modulated frequency. Thus, a larger, higher-speed optical communications apparatus can be achieved.
According to a seventh aspect, in the first aspect,
the variable wavelength optical modulator carries out optical frequency modulation,
each of the RF optical routers includes:
an optical brancher for outputting the optical signal from a plurality of output terminals; and
a plurality of optical filters each for suppressing, on the optical signal outputted from the corresponding output terminal of the optical brancher, any one of an upper sideband component and a lower sideband component corresponding to the predetermined frequency of the RF modulated signal, and
each of the optical receivers includes:
a square-law-detector for carrying out square-law-detection on the optical signal outputted from the corresponding RF optical router, and outputting the RF modulated signal.
In the seventh aspect, optical intensity modulation is used as the optical modulation scheme. In the optical signal, any one of the double sidebands corresponding to the RF modulated signal to be extracted is suppressed. Then, the optical signal is square-detected by the optical receiving terminal, and then the RF modulated signal is reproduced for routing in the optical domain based on the RF modulated frequency. Thus, a larger, higher-speed optical communications apparatus can be achieved.
An eighth aspect of the present invention is directed to an optical communications method for optically transmitting a transmission signal including data information to a destination, and the method includes:
a variable frequency RF modulating step of modulating the transmission signal into an RF modulated signal with a predetermined carrier frequency that uniquely corresponds to the destination in a predetermined group to which the destination belongs;
a variable wavelength optical modulating step of modulating the RF modulated signal outputted from the variable frequency RF modulator into an optical signal with a predetermined optical wavelength that uniquely corresponds to the predetermined group to which the destination belongs;
an optical routing step of selecting a distribution route corresponding to the wavelength of the optical signal converted in the variable wavelength optical modulating step, and outputting the optical signal to the distribution route;
an RF optical routing step of selecting an end route corresponding to the carrier frequency of the RF modulated signal of the optical signal outputted in the optical routing step, and outputting the optical signal to the end route; and
an optical receiving step of converting the optical signal outputted in the RF optical routing step into an electrical signal that corresponds to the transmission signal.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.